Internal combustion engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art, may emit exhaust containing a complex mixture of solid, liquid, and gaseous components. For example, the gaseous components of the exhaust may include compounds such as nitrous oxides (“NOx”) and CO, and the solid and/or liquid components of the exhaust may include soluble organic fraction, soot, and/or unburned hydrocarbons. Together, the soluble organic fraction, soot, and unburned hydrocarbons emitted by internal combustion engines are generally referred to as “particulate matter.”
The government regulates the exhaust released into the atmosphere from such engines based on the type, size, and/or class of engine. These exhaust regulations continue to become more stringent, and engine manufacturers typically use catalytic exhaust treatment systems to comply with these regulations. In such systems, a reductant, such as urea or ammonia, may be injected into the exhaust upstream of a selective catalytic reduction (“SCR”) catalyst, and the catalyst materials within the SCR catalyst may reduce NOx carried by the exhaust in the presence of the reductant. In addition, a particulate filter may capture a portion of the particulate matter carried by the exhaust.
The effectiveness of an SCR catalyst is based on its ability to convert NOx carried in the exhaust to N2 and other gaseous species such as O2 and H2O. Maintaining the SCR catalyst within a desired temperature range and providing it with a flow of exhaust having a sufficient level of NO2 are both factors that assist in maximizing the NOx conversion efficiency of the SCR catalyst. The exhaust leaving the engine, however, typically has a temperature well above the optimum temperature range of the SCR catalyst. Generally, such exhaust also has levels of NO2 far below the range needed for optimum NOx reduction by the SCR catalyst. Thus, exhaust treatment systems often include an oxidation catalyst disposed upstream of the SCR catalyst to assist in oxidizing the relatively abundant NO present in the exhaust. Oxidizing the NO may increase the amount of NO2 present in the exhaust entering the SCR catalyst, and may assist in maximizing the conversion of NOx by the SCR catalyst.
An exemplary exhaust treatment system for controlling the NOx and particulate matter emissions of an internal combustion engine is illustrated in U.S. Pat. No. 6,928,806 (“the '806 patent”). Specifically, the '806 patent discloses an oxidation catalyst, an SCR catalyst coupled downstream of an oxidation catalyst, and a particulate filter coupled downstream of the SCR catalyst. The disclosed oxidation catalyst increases the level of NO2 entering the SCR catalyst, and the particulate filter removes particulates from the exhaust before the exhaust is released to the atmosphere.
Although the system disclosed in the '806 patent may assist in removing particulate matter and reducing the NOx content of the exhaust, the system of the '806 patent has several drawbacks. For example, due to the dynamic nature of the exhaust temperature and flow rate, it can be difficult to maintain operation of the SCR catalyst employed by the '806 patent within its optimal temperature and NO2 levels. Moreover, the SCR catalyst and particulate filter employed by the '806 patent are large and difficult to remove or replace for servicing. The size of these components also makes it difficult to package the exhaust treatment system in, for example, the engine compartment of a vehicle or other machine where space is limited. In addition, such particulate filters require frequent on-vehicle regeneration using various heating devices. Such heating devices are difficult to use, and further increase the cost and complexity of the exhaust treatment system. Moreover, actively regenerating the particulate filter using such heating devices requires heating the exhaust to temperatures that can be harmful to the catalyst materials used in the oxidation and SCR catalysts.
The embodiments of the present disclosure are directed toward overcoming the deficiencies described above.